After drilling a wellbore into a hydrocarbon-bearing formation, the well is completed in preparation for production. To complete a well, a casing (liner), generally steel, is inserted into the wellbore. Once the casing is inserted into the wellbore, it is then cemented in place, by pumping cement into the gap between the casing and the borehole (annulus). The reasons for doing this are many, but essentially, the casing helps ensure the integrity of the wellbore, i.e., so that it does not collapse. Another reason for the wellbore casing is to isolate different geologic zones, e.g., an oil-bearing zone from an undesirable water-bearing zone. By placing casing in the wellbore and cementing the casing to the wellbore, then selectively placing holes in the casing, one can effectively isolate certain portions of the subsurface, for instance to avoid the co-production of water along with oil.
The process of selectively placing holes in the casing and cement so that oil and gas can flow from the formation into the wellbore and eventually to the surface is generally known as “perforating.” One common way to do this is to lower a perforating gun into the wellbore using a wireline or slickline cable to the desired depth, then detonate a shaped charge mounted on the main body of the gun. The shaped charge creates a hole in the adjacent wellbore casing and the formation behind the casing. This hole is known as a “perforation”. U.S. Pat. No. 5,816,343, assigned to Schlumberger Technology Corporation, incorporated by reference in its entirety, discusses prior art perforating systems.
In order to optimize the performance of perforated completions, it is necessary to know the details of the completion behaviour. For example, it is beneficial to know which perforations are flowing and which are not due to conditions such as formation debris blockage or tunnel collapse. Additionally, it is beneficial to know what fluids are flowing from the individual perforations and which tunnels are producing sand as well as hydrocarbons. If the behavioural details of the individual perforations are known, then treatments for detrimental conditions can be appropriately applied.
Related oilfield technology exists in a number of areas. For example, for open hole sections of the well, images are frequently acquired using tools such as the Ultrasonic Borehole Imager (i.e., acoustic pulses), the Formation Microscanner (i.e., electrical resistivity) or the GeoVision resistivity tool. However, these devices are not applicable to cased hole environments.
In cased holes, Kinley calipers or similar tools are used to form maps of damage or holes in casing by using mechanical feelers as the sensing elements. Downhole video cameras can also be used to view perforations in cased holes, but the well must be shut-in (or very nearly shut-in) and filled with filtered fluid for the cameras to be effective. Temperature logs and production logging tools can be used in cased holes but have no azimuthal sensitivity and insufficient depth resolution to detect problems with individual perforations.
There exists, therefore, a need to see the behaviour of individual perforations in a cased hole.